1. Technical Field
This invention relates to a light-emitting layer for use in inorganic EL devices, and more particularly, to a white light emitting multilayer of phosphor thin films and an EL panel using the same.
2. Background Art
In the recent years, active research works have been made on thin-film EL devices as small-size and large-size, lightweight flat displays. A monochromatic thin-film EL display using a phosphor thin film of manganese-doped zinc sulfide capable of emitting yellowish orange light has already become commercially practical as a dual insulated structure using thin-film insulating layers 2 and 4 as shown in FIG. 2. In FIG. 2, a predetermined pattern of lower electrodes 5 is formed on a substrate 1, and a first insulating layer 2 is formed on the lower electrodes 5. On the first insulating layer 2, a light-emitting layer 3 and a second insulating layer 4 are successively formed. On the second insulating layer 4, a predetermined pattern of upper electrodes 6 is formed so as to construct a matrix circuit with the lower electrodes 5.
Thin-film EL displays must display images in color in order that they find use as computer, TV and similar monitors. Thin-film EL displays using sulfide phosphor thin films are fully reliable and resistant to environment, but at present regarded unsuitable as color displays because EL phosphors required to emit light in the primaries of red, green and blue have poor characteristics. Engineers continued research on SrS:Ce (using SrS as a matrix material and Ce as a luminescent center) and ZnS:Tm as a candidate for the blue light-emitting phosphor, ZnS:Sm and CaS:Eu as a candidate for the red light-emitting phosphor, and ZnS:Tb and CaS:Ce as a candidate for the green light-emitting phosphor.
These phosphor thin films capable of emitting light in the primaries of red, green and blue suffer from problems of emission luminance, emission efficiency and color purity. Thus color EL panels have not reached the commercial stage. Referring to the blue color among others, a relatively high luminance is achieved using SrS:Ce. However, its luminance is still short as the blue color for full-color displays and its chromaticity is shifted toward green. There is a desire to have a better blue light-emitting layer.
To solve the above problem, thiogallate and thioaluminate base blue phosphors such as SrGa2S4:Ce, CaGa2S4:Ce, and BaAl2S4:Eu were developed as described in JP-A 7-122364, JP-A 8-134440, Shinshu Univ. Technical Report, EID 98-113, pp. 19-24, and Jpn. J. Appl. Phys., Vol. 38 (1999), pp. L1291-1292. These thiogallate base phosphors are satisfactory in color purity, but suffer from a low luminance and especially, difficulty to form a thin film of uniform composition because of the multi-component composition. It is believed that thin films of quality are not obtainable because of poor crystallinity resulting from inconvenient composition control, formation of defects resulting from sulfur removal, and admittance of impurities; and these factors lead to a failure to increase the luminance. In particular, thioaluminate base phosphors are quite difficult to control their composition.
On the other hand, monochromatic displays using orange phosphor ZnS:Mn have already become commercially practical. The visibility of displayed images imposes a need for white mono-color. To meet the need, a variety of white phosphors have been studied as described in Display and Imaging, Vol. 3 (1994), pp. 159-171.
In the prior art, ZnS:Pr thin film having praseodymium added as a luminescent center is known as the white phosphor. Since the spectrum of light emission consists of bright lines, this thin film develops a thin film interference effect. There still remain problems of visual angle dependency and a low luminance. Meanwhile, a variety of white phosphors have been developed as by starting with blue-green phosphor of SrS:Ce and adding a red component such as europium or manganese thereto.
For example, there are known:
(1) a method of adding both Ce and Eu as the luminescent center to SrS matrix material to form SrS:Ce, Eu,
(2) a method of using a multilayer structure of SrS:Ce and SrS:Eu to form SrS:Ce/SrS:Eu,
(3) a method of using a multilayer structure of SrS:Ce and CaS:Eu to form SrS:Ce/CaS:Eu, and
(4) a method of using a multilayer structure of SrS:Ce and ZnS:Mn to form SrS:Ce/ZnS:Mn.
These white phosphor thin films based on SrS:Ce emit a broad spectrum of light and are ideal for white light emission, but provide a low luminance and a white color close to yellow, known as egg-shell white. From the standpoint of human factor engineering, a color close to paper white, that is, CIE chromaticity coordinates of x=0.3 and y=0.3 has not been available unless filters are used.
An object of the invention is to provide a phosphor multilayer eliminating a need for filters, having a satisfactory color purity and suitable for use in paper white mono-color EL panels, and an EL panel.
This and other objects are attained by the present invention which is defined below as (1) to (4).
(1) A phosphor multilayer comprising at least a first thin film and a second thin film, wherein
said first thin film is formed of a matrix material comprising barium aluminate as a main component, containing sulfur element and further containing europium as a luminescent center, and
said second thin film is formed of a matrix material comprising zinc sulfide as a main component.
(2) The phosphor multilayer of (1) wherein the molar ratio of the content of sulfur element admixed to the content of oxygen element in the matrix material, as expressed by S/(S+O), is between 0.02 and 0.5.
(3) The phosphor multilayer of (1) wherein the molar ratio of the content of sulfur element admixed to the content of oxygen element in the matrix material, as expressed by S/(S+O), is between 0.7 and 0.9, and the ratio of aluminum element Al to barium element Ba, as expressed by Al/Ba, is between 1.5 and 3.0.
(4) The phosphor multilayer of (1) or (2) wherein the synthetic color of emissions from said first and second thin films is white having CIE chromaticity coordinates of x=0.27 to 0.39 and y=0.27 to 0.38.
(5) An EL panel having the phosphor multilayer of any one of (1) to (4).